This invention relates to the field of printed circuit board design and manufacture, and more particularly to a printed circuit board having a universal trace, pad and hole pattern which allows the board to be manufactured virtually to completion before the manufacturer even obtains the masks to create the trace pattern for a particular circuit.
Printed circuit boards are used in a wide variety of electronic devices. The boards serve to support the electronic circuit components of the devices while "printed" filaments of conductive material (called traces) on the surface of the insulating board substrate, supply power to and interconnect the circuit components mounted on the board surface. In multilayer boards, some of the traces run between insulating layers in the interior of the board.
Printed traces perform the same function as wires but have several advantages over wire. For example, since the traces are printed on the board, they are considerably less bulky than wire. Printed traces also eliminate all of the labor that is required to interconnect electronic components with wire, such as cutting the wire to appropriate lengths, stripping insulation off the wire, soldering individual wires to component leads, etc.
In addition to traces, conventional printed circuit boards also have "pads" and "holes." A pad is an area of conductive material which is printed on the board surface and to which a component lead is attached. Each pad is coated with a small quantity of solder. Each pad is also usually centered around a hole which is drilled through the board. The holes receive the component leads and, when melted and then resolidified, the solder on the pads electrically and mechanically connects the leads to the pads. The traces, of course, electrically interconnect the pads to form a working circuit.
A variety of techniques exist for making printed circuit boards. These previously known techniques utilize partially transparent optical masks containing an image of the desired pads and traces. Through a series of production steps this image is transferred to the board and embodied in the form of conductive copper pads and traces on the insulating board substrate. However, since the traces must interconnect the component leads in the proper fashion to form a functioning circuit, these previous techniques require that the circuit to be imaged on the board be completely designed and that the position of the components on the board be determined before the mask can be created. Thus, except for a few of the very earliest steps toward manufacturing the completed board, such as forming an insulating substrate, virtually all manufacturing steps toward the completion of previously known printed circuit boards must wait until the particular circuit and component positions are determined so that a mask having the correct pad and trace image can be created.
In practice, the delay of these manufacturing steps frequently results in a rush to produce prototype boards in order to keep up with production schedules. Furthermore, the small quantity of prototype boards required to test the circuit design, coupled with the premium rates for fast service can increase the cost of conventional prototype boards by factors of ten or more over normal printed circuit board production costs.
Once the design of the device passes the prototype stage, conventional printed circuit boards are then usually manufactured in large quantities in advance of the time required as a way to reduce costs. However, errors in the design of the board, a change in design concept or lack of a previously available component can render existing inventories useless since the conventional boards are specifically designed for only one particular circuit and layout of electronic components.
Although a number of different methods for producing conventional printed circuit boards are now in use, two methods predominate. The first method, which is called the "print and etch" method, proceeds generally as follows:
The first step is to make a printed circuit board blank by covering a board made of insulating material with thin copper foil. The insulating board is generally, but not necessarily, made of a laminated epoxy and fiberglass composition which is frequently referred to in the printed circuit board industry as "laminate." With many previously known printed circuit board production techniques, producing the laminate blanks was the only step in the production process which a manufacturer could perform before obtaining the printed circuit masks.
After the manufacturer obtains these "artwork" masks having the unique pad and trace image for a particular circuit and component layout, he can then drill holes in the blank at the center of each pad location. However, friction caused by drilling melts the epoxy resin of the laminate around the hole. This melted epoxy resin may be removed with an etching solution in a process known in the printed circuit board industry as "smear removal."
For multilayer boards, smear removal is particularly important since it exposes the edges of certain interior conductive traces positioned at the circumference of particular holes. The interior walls of the holes are then plated with copper in an electroless plating process so that the electroless copper plating electrically connects the inner layer traces, which abut the edges of the the holes, to the exterior traces abutting the same holes. In this way, the drilling, smear removal and electroless plating processes electrically connect components without having traces use up space on the surface of the board. This foil coated board is then electroplated with additional copper until a desired thickness of copper is built up on the board and the walls of the holes.
The resulting electroplated board is then coated with photo-resist. A partly transparent optical mask containing a negative image of the pads and circuit traces is placed on the photo-resist coated surface of the board. The walls of the holes and those portions of the board which will form the pads and traces are then exposed to ultraviolet light through the transparent portions of the mask. The ultraviolet light polymerizes the photo-resist. The remainder of the board surface is protected from exposure to the ultraviolet light by the opaque portions of the mask. Unpolymerized photo-resist under these opaque portions is then stripped away by a developing solution capable of removing unpolymerized photo-resist. This solution does not, however, affect the exposed polymerized portions. The resulting circuit board has a positive photo-resist image of the pads and traces printed thereon. This process for creating a photo-resist image of the pads and traces on the circuit board blank is called "imaging."
The imaged circuit board is then placed in an etching bath which etches away the unprotected copper. Since the etching solution in the bath does not attack the polymerized photo-resist, the pads, the plated hole walls and the traces protected by the polymerized photo-resist remain on the board. The resulting board is then immersed in a solution which chemically strips away the polymerized photo-resist, leaving bare copper traces, pads, and plated holes.
The next step is to cover all of the traces with a coating of "solder-mask." In the solder-masking process, virtually the entire surface of the board is coated with solder-mask material, including the exposed portions of the insulating board substrate as well as the traces. Only the pads and the walls of the holes remain bare.
The board is then dipped in a tank of molten solder. Molten solder, however, will not adhere to the solder-mask. Thus, when the board is removed from the tank, only the pads and holes are coated with solder. To prevent the solder from plugging the holes, a blast of hot air is directed against the surface of the board. The air blast blows excess solder out of the holes.
In later production steps, when electronic components are attached to the board, the solder on the pads is remelted and then resolidified around the component leads to electrically and mechanically connect the electronic components to the printed circuit board. Additional solder may be supplied to the pads and leads during this step and the coating of solder-mask over the traces prevents this additional solder from shorting out adjacent traces.
Since the solder-mask protects the traces during the soldering process, it must be both insulating and, unlike photo-resist, it must also be resistant to the high temperature of molten solder. Several different kinds of solder-mask exist. A commonly used solder mask, called Vacrel ,.sup.1 is polymerized with ultraviolet light and is thus used in the same manner as conventional photo-resist. However, other types of non-photosensitive solder-mask may be silk screened onto the surface of the board so that the solder-mask material coats the entire surface of the board except for the pads. FNT .sup.1 Vacrel is a registered trademark of DuPont
A second common method for producing conventional printed circuit boards is called the "pattern plate" method. Although the same general steps are used in both the pattern plate and print and etch methods, and although both methods achieve the same end result, the pattern plating process performs some of the steps in a slightly different way and in a different order.
As with the print and etch method, the pattern plate method starts with a copper foil covered insulating board. The manufacturer must then obtain the optical mask having the unique pad and trace image for the particular circuit and component layout. Holes are then drilled through the board at the center of each pad position, the smear removed and the holes plated with electroless copper.
The resulting copper clad board is coated with photo-resist. As with the print and etch method, a partly transparent optical mask is positioned between the photo-resist coated surface and an ultraviolet light source. However, in contradistinction to the negative trace image of the print and etch mask, the mask used with the pattern plate method contains a positive image of the printed circuit board pads and traces. Thus, those portions of the board which will eventially be etched away are exposed through the mask to polymerize the photo-resist coating. The unpolymerized photo-resist, which is protected from exposure by the opaque portions of the mask, covers those portions of the board which are intended to form the pads and traces. The opaque portions of the mask also protect the walls of the holes from exposure to the ultraviolet light. The unexposed photo-resist covering the pads, traces and hole walls is then dissolved away by the same type of developing solution used in the print and etch method. At this step in the process, the coating of polymerized photo-resist remaining on the printed circuit board blank is a negative image of the pads and traces, as opposed to the positive image formed at the analogous step in the print and etch process.
The bare copper pads, traces and the walls of the holes are subsequently electroplated with an additional layer of copper to a desired thickness and then further electroplated with a mixture of tin and lead. Then, the polymerized photo-resist is chemically removed. The unprotected copper is then etched away. The etching solution which etches away the unprotected copper does not attack the tin-lead plating. Thus, the tin-lead plating over the pads, the walls of the holes and traces acts like the polymerized photo-resist in other steps and protects the underlying copper from dissolution.
The resulting board is then immersed in a chemical stripping solution which removes the tin-lead plating mixture from the copper pads, the walls of the holes and traces. Now the solder-masking procedure can take place exactly as it did in the print and etch technique. That is, the entire surface of the board, except for the pads and hole walls, is coated with solder-mask. Then, the board is dipped in molten solder to coat the pads and the walls of the holes with solder. The traces are not coated with solder since solder will not stick to the protective solder-mask coating. As in the print and etch method, a blast of hot air blows excess molten solder out of the holes.
As can be seen from the above description of conventional printed circuit board production techniques, the imaging steps are performed early on in the processes. Thus, before the vast majority of production steps can be performed, the manufacturer must obtain masks defining the layout of the printed circuit board traces and the position of the pads. This results in a long production time from the completion of a circuit design and creation of a printed circuit board mask to the provision of a finished circuit board product.
In contrast, the printed circuit board and method of printed circuit board manufacture of the present invention allows the printed circuit board manufacturer to perform virtually all manufacturing steps toward production of a completed board before completion of the printed circuit board masks. In fact, the boards of the present invention can be manufactured virtually to completion even before a circuit designer conceives of the particular circuit to be placed on the board.
Furthermore, since, with the present invention, it is not necessary to create large inventories of boards printed for a particular circuit, electronic device manufacturers can modify circuits at any time after the device is in production without sacrificing large inventories of boards. Thus, the present invention eliminates many of the drawbacks of prior known technology with respect to the time interval from when the manufacturer receives the mask to completion of prototype boards and the lack of flexibility in modifying printed circuit board inventories once the traces, pads and holes have been formed on conventional boards.